Modern integrated circuit (IC) chips typically contain millions of transistors on a single chip. With the speed of some IC chips operating at over a gigahertz (GHz), one of the persistent problems facing IC chip design is the adverse effects of temperature on the performance of the devices (e.g., transistors, resistors, etc.) embedded in the chip. For example, the devices in the IC chips typically produce heat during operation of the circuit. In some cases, the heat generated from the devices can increase the overall temperature of the chip in the order of several hundred degrees.
The temperature increase in an IC chip environment adversely affects the performance of the circuit. For example, a temperature increase typically increases switching time of transistors in an IC chip because electrical conductivity is inversely proportional to temperature. That is, a higher temperature usually slows down the speed of devices in the integrated chip whereas a lower temperature speeds up the devices.
In addition to temperature variations, integrated circuits may also suffer from process variations. One of the main causes of the process variation is non-uniform base-emitter voltages of bipolar junction transistors. For example, during manufacturing processes, non-uniform doping concentrations may produce bipolar junction transistors that have varying base-emitter voltages. Higher base-emitter voltages speed up and improve conductivity of the transistors while lower threshold voltages slow down and reduce conductivity of the transistors. The variation in the process therefore introduces variation in speed and conductivity of transistors in integrated circuits.
In general, the temperature and process variations affect IC chips that run at high speeds more than chips that operate at lower speeds. This is because devices operating at a high switching speed produce more heat than the devices running at a lower speed. Thus, high speed IC chips are more sensitive to variations in temperature and process.
Since the invention of the IC devices, the speed of IC chips has been steadily increasing by implementing faster switching circuits. One of the fastest IC is emitter-coupled logic (ECL) circuit with propagation delay of less than 1 nanosecond and clock rates over 1 GHz. ECL circuits are often used in high speed digital logic circuits. To operate at such high speeds, the differential ECL circuit requires low capacitive load at load resistors to avoid parasitic filtering. Unfortunately, the differential ECL circuit is typically highly sensitive to variations in temperature and process. In particular, the DC output level of the ECL circuit changes with the variation in temperature and process.
Prior Art FIG. 1 illustrates a conventional temperature compensation circuit 102 for compensating temperature variations in an ECL circuit 100 through DC level compensation. The ECL circuit 100 comprises transistors N1, N2, N3, N4, N5, and N6, load resistors 106 and 108, an emitter series feedback resistor 110, and a reference voltage network 104. The ECL circuit 100 receives two input signals I1 and I2 at the base of the transistors N1 and N2, respectively. The reference voltage network 104 provides a bias voltage to the bases of the transistors N3 and N4. The transistors N5 and N6 form output follower transistors for providing outputs O1 and O2 at the respective emitters.
The temperature compensation circuit 102 comprises a pair of diodes D1 and D2 coupled in parallel and a resistor 112. The resistor is coupled in series to the parallel coupled diodes D1 and D2. In this setting, the diodes D1 and D2 operate to allow current in one direction at a time between junctions 116 and 118. The compensation circuit 102 provides DC level compensation to the output transistors N5 and N6 by adjusting current flow through the load resistors 106 and 108.
Unfortunately, the temperature compensation circuit 102 presents several drawbacks. For example, the compensation circuit also affects the AC operations of the ECL circuit 100 with a resulting adverse effect on the high speed signal. Furthermore, the compensation circuit 102 causes a large capacitive load due to a large current that can flow through the diodes D1 and D2. This means that the diodes D1 and D2 must be large enough to handle such large current. This, in turn, leads to bandwidth reduction. In addition, the compensation circuit 102 is not operative to start compensation at a specified temperature and is generally not process independent since the circuit is not symmetrical.
Thus, what is needed is a DC level compensating circuit that provides a substantially constant DC voltage level at the output transistor of a differential ECL circuit by compensating for variations in temperature and process without affecting AC operation. What is also needed is a compensation circuit having low capacitive load to allow high frequency applications in ECL circuits.